1. Field
The present invention relates to an image forming apparatus and a process cartridge.
2. Related Art
In a typical electrophotographic image forming apparatus, residual toner remaining on the image bearing member is removed by a cleaning member after a toner image is transferred to a transfer medium or an intermediate transfer element.
Typically, a reed-like cleaning blade is used as such a cleaning member because it has a simple structure and demonstrates good cleaning performance.
Such a cleaning blade is constituted of an elastic material such as polyurethane rubber.
While the base end of the cleaning blade is supported by a supporting member, the front end of the cleaning blade is pressed against the surface of the image bearing member to scrape off the toner remaining on the image bearing member.
Recently, image forming apparatuses using a toner consisting of particles having a nearly spherical form of small particle diameter manufactured by a polymerization method, etc., have been introduced to meet market demand for improvement of image quality.
This polymerized toner has excellent transfer efficiency in comparison with typical toner and is capable of satisfying this demand.
However, the same qualities that make polymerized toner superior to conventional toner also make it difficult to remove the toner from the surface of the image bearing member sufficiently with the cleaning blade.
This problem occurs because the polymerized toner having a nearly spherical form and such a small particle diameter easily slips through a minute gap appearing between the edge of the cleaning blade and the surface of the image bearing member.
One way to prevent such slip-through of the toner is to increase the contact pressure between the image bearing member and the cleaning blade.
However, as described in JP-2010-191378-A and as illustrated in FIG. 8A, by increasing the contact pressure of the cleaning blade, the friction between an image bearing member 123 and a cleaning blade 62 increases so that the cleaning blade 62 is pulled in the moving direction of the image bearing member 123, resulting in turning inward or outward of a front edge portion 62c of the cleaning blade 62.
The cleaning blade 62 may then creak when it tries to reacquire its original shape against the turning inward or outward.
Furthermore, if cleaning continues with the front edge portion 62c turned inward or outward, the cleaning blade 62 is disproportionally abraded at several μm from the front edge portion 62c of a front end surface 62a of the cleaning blade 62 as illustrated in FIG. 8B. If this cleaning still continues, the disproportionally abraded portion increases, and finally leads to chipping-off of the front edge portion 62c as illustrated in FIG. 8C.
Once part of the front edge portion 62c has chipped off, the cleaning blade is no longer capable of removing the toner properly, resulting in poor cleaning performance.
JP-2010-191378-A mentioned above describes a cleaning blade that includes an elastic blade having a low-friction surface made by impregnation of a surface layer with at least one of an isocyanate compound, a fluorine-containing compound, and a silicone compound.
The surface layer covers the front edge portion of the elastic blade and is formed of an ultraviolet-cured resin harder than the elastic blade.
Since the hardness of the front edge portion increases because of the surface layer harder than the elastic blade, it is possible to prevent deformation of the front edge portion in the surface moving direction of the image bearing member.
In addition, if the surface layer is abraded over an extended period of time of use and the front edge portion of the elastic blade is exposed, the impregnated portion of the elastic blade contacts the surface of the image bearing member, thereby reducing the friction between the elastic blade and the image bearing member, which contributes to prevention of deformation of the exposed portion.
In addition, the turning inward and outward of the front edge portion is also reduced and the abrasion resistance of the cleaning blade ameliorates, so that poor cleaning performance is prevented over an extended period of time.
There are other cleaning blades having a hard front edge portion by providing a hard surface layer to the elastic blade.
For example, JP-3602898-B1 (JP-H09-127846-A) describes such a cleaning blade having an elastic blade having a front edge portion covered with a surface layer formed of a resin having a film hardness equivalent to a pencil hardness of from B to 6H. JP-2004-233818-A describes a cleaning blade having an elastic blade having a surface layer harder than the elastic blade, which is provided at least at portions that contact the image bearing member.
The surface layer is formed by impregnating and expanding the elastic blade with an ultraviolet curing material containing silicone followed by ultraviolet irradiation treatment.
However, these cleaning blades having the hardened front edge portions described above find it difficult to sustain good cleaning performance under difficult conditions such as continuous solid image printing, during which an extremely large amount of powder (toner) is attached to the image bearing member.
The inferred failure mechanism is as follows:
The surface layer and the impregnated portion are provided in the longitudinal direction of the front end surface of the elastic blade, which may have an adverse impact on the elasticity of the elastic blade.
If the elasticity of the elastic blade is adversely affected and the image bearing member is decentered or has minute waviness on the surface, the contact pressure of the cleaning blade with the surface of the image bearing member varies in the longitudinal direction.
Consequently, the front edge portion of the cleaning blade becomes unable to maintain suitable contact with the surface of the image bearing member.
When the cleaning blade collects a large quantity of toner while the image forming apparatus is forming solid images continuously, the pressure on the cleaning blade from the toner collected at the cleaning blade increases.
Therefore, if the pressure exerted by the toner on the image bearing member on the cleaning blade surpasses the contact pressure of the cleaning blade, the contact is not sustained at the portions where the contact pressure of the cleaning blade to the image bearing member is low, resulting in slip-through of the toner.
As a result, poor cleaning performance occurs under difficult conditions like continuous solid image printing.
In the structure in which a surface layer harder than an elastic blade is formed after impregnating the urethane rubber elastic blade with an isocyanate compound as described for example in JP-2010-191378-A mentioned above, the isocyanate compound reacts chemically with urethane rubber, so that the cross-linking density of the impregnated portion increases. By having such an impregnated portion, the elastic blade loses its elasticity, which leads to deterioration in the ability of the elastic blade to maintain contact with the image bearing member because of the eccentricity thereof, etc.
Consequently, maintaining good cleaning performance is difficult.
In the structure in which the resin having a pencil hardness of from B to 6H is provided to form the surface layer described in JP-3602898-B1 (JP-H09-127846-A) mentioned above, the abrasion resistance of the surface layer is not sufficient, resulting in quick disappearance of the surface layer due to the abrasion with the image bearing member.
If the surface layer is made thicker, the elasticity of the elastic blade decreases and the ability of the blade to maintain contact with the image bearing member in the face of the variation caused by eccentricity of the image bearing member, etc. deteriorates, resulting in poor cleaning performance.
In addition, the structure described in JP-2004-233818-A mentioned above, which is formed by impregnating and expanding the elastic blade with an ultraviolet curing material containing silicone followed by ultraviolet irradiation treatment to provide a surface layer harder than the elastic blade to the contact portion with the image bearing member, requires impregnation with a large amount of the ultraviolet curing material to sufficiently harden the surface layer.
However, if the elastic blade is impregnated with a large amount of the ultraviolet curing material, the ultraviolet curing material inside the elastic blade increases, thereby forming an excessively hard and deep impregnated portion, which leads to deterioration of the elasticity of the elastic blade.
Consequently, the ability of the front edge portion of the blade to maintain contact with the surface of the image bearing member deteriorates, resulting in poor cleaning performance.
In addition, with a cleaning blade having a front edge portion harder than the elastic blade, the surface of the image bearing member tends to be abraded sooner than with an elastic blade, which leads to background fouling and other image quality problems.
JP-2010-191378-A mentioned above also describes an image forming apparatus using an image bearing member having a surface layer formed of a cross-linking type charge transport material.
The cleaning blade having a front edge portion harder than the elastic blade contacts the image bearing member.
However, this image forming apparatus is just a combination of the cleaning blade and the image bearing member, both of which have improved mechanical durability.
Since contact between the hard layers with friction causes abrasion of the cleaning blade and/or the image bearing member, cleaning performance deteriorates over an extended period of time.
Furthermore, since the cleaning blade having a hard front edge portion presses toner against the surface of the image bearing member and the surface of the image bearing member is hard to wear down, silica particles externally added to the toner are attached to and fixed on the surface of the image bearing member due to the pressure and friction heat generated by such hard contact, which is referred to as filming of the image bearing member.
Filming of the image bearing member leads directly to production of defective images.